1. Field of the Invention
The present invention relates to a solid-state image pickup device and an image sensor using this device, and more particularly, it relates to a solid-state image pickup device whose resolution is lowered and which reads a signal at a high speed, and an image sensor using this device.
2. Description of the Related Art
In a one-dimensional image sensor for use in reading an image in a scanner and the like, or a two-dimensional image sensor for use in reading a two-dimensional image of a digital camera, a camera type cellular phone or the like, a solid-state image pickup device has been used. In this solid-state image pickup device, a constitution has been proposed in which an output of a charge detection section disposed in a photosensitive pixel is selected and output when reading an electric charge generated in the photosensitive pixel.
FIG. 25 shows a schematic constitution of a conventional output selection type solid-state image pickup device. In FIG. 25, an image sensor includes photosensitive pixels 1 which convert received light energy into a signal charges by photoelectric conversion, and charge detection sections 2 which detect potentials of the signal charges of the photosensitive pixels 1. Each of the charge detection sections 2 includes a floating diffusion layer 3 in which the electric charge is temporarily stored, a reset gate 4 which sets the potential of the floating diffusion layer 3, and a drain 5 to which a reference voltage is applied via the reset gate 4. The image sensor further includes buffers 6 which convert potential changes of the floating diffusion layers 3 into signal outputs, transistors 7 which select signal outputs of the buffers 6, a shift register 8 which outputs pulses RS for setting the potentials of the floating diffusion layers 3 to the reset gates 4 and which outputs selection pulses SL to the transistors 7, and a buffer 9 which holds a final output.
An operation in the above-described constitution will be described. The electric charges generated in the photosensitive pixels 1 are passed into the floating diffusion layers 3 of the charge detection sections 2, and the potential changes are converted into signal outputs by the buffers 6. The transistor 7 is selectively turned on by a selection pulse (SLi) generated in the shift register 8, the output is selected, and an output signal (OS) is finally output via the buffer 9. To set the potentials of the floating diffusion layers 3, a pulse (RSi) is applied to the reset gate 4 in such a manner that the drain 5 to which the reference voltage has been applied conducts.
Additionally, according to the constitution of the conventional solid-state image pickup device shown in FIG. 25, a signal storage time of the i-th photosensitive pixel 1, and that of the i+1-th photosensitive pixel 1 differ with each pixel as shown by arrows Pa and Pb shown in FIG. 26, and restrictions on use are sometimes generated. When the stored electric charge of a certain pixel is read as the signal, another pixel stores the signal in some case. Therefore, for example, when LEDs of three colors of red (R), green (G), and blue (B) are alternately lit as color light sources, a lighting time needs to be separated from a reading time, and there has been a problem that the whole reading time lengthens. The operation at this time is shown in waveform diagrams of FIGS. 27(a) to (d). For example, a time when each light source lights by LED_R of FIG. 27(a) and LED_G of FIG. 27(b), and a time when the signal is read in FIG. 27(d) are required for a period shown by arrows for which “G-integration” is described in the figure.
As a constitution in which simultaneous reading is possible in the conventional solid-state image pickup device shown in FIG. 25, a switching transistor 10 for switching and a capacitance 11 for holding the signal may be disposed between the buffer 6 for converting the potential change of the floating diffusion layer 3 into the signal output and the transistor 7 which is selectively turned on by the selection pulse generated by the shift register 8 in order to hold the signal as in a solid-state image pickup device in a different conventional example shown in FIG. 28. However, according to the solid-state image pickup device by the other conventional example shown in FIG. 28, there has been a problem that noises increase or a circuit area increases by the switching transistor 10.
According to the conventional solid-state image pickup devices described above, there has been a problem that a fluctuation is generated in the signal storage time for each photosensitive pixel to cause the restrictions on the use, or that the total reading time lengthens in a case where the lighting time has to be separated from the reading time. In the constitution in which the capacitance for holding the signal and the switching transistor are disposed in order to allow the simultaneous reading, there has been a problem that the increase of the noise or the circuit area is caused.